This invention relates to a method for preparing a porous polyethylene object of desired shape. Said polyethylene has a molecular weight in excess of 1,000,000. A polyethylene with such a molecular weight is often referred to as an ultra high molecular weight polyethylene. The porous structure has many utilities depending on its shape, among them filter funnels, immersion filters, filter crucibles, porous sheets, etc.
This polyethylene has certain properties superior to a similar polyethylene with a lower molecular weight. These superior physical properties are: toughness, impact strength, abrasion resistance, antifriction, good performance at temperatures below freezing and excellent stability at temperature as high as 300.degree.F. But because this polyethylene is extremely viscous above its melting point, it sustains substantial shear degradation if mechanical force, for example by extrusion, calendering or injection molding, is applied to shape the molten polyethylene. Thus shaping, by extrusion, etc., causes the resultant polyethylene structure to have physical properties which are less than the original properties of the unshaped polyethylene. However, the superior physical properties of this polyethylene are maintained by the porous polyethylene objects prepared by the method described herein.
J. E. Kepple, U.S. Pat. No. 3,308,073, dated Mar. 7, 1967, teaches a method of making porous objects from low molecular weight polyethylene and normally liquid hydrocarbons. In this method 92.5 parts of isoparaffinic hydrocarbon fraction with a boiling range of 420.degree.-475.degree.F and 7.5 parts of polyethylene with a molecular weight of about 290,000 were heated together to above the fusion temperature of the polyethylene. The resulting viscous homogeneous liquid could be poured from the containing beaker. By comparison if 92.5 parts of isoparaffin hydrocarbon fraction with a boiling range of 420.degree.-475.degree.F and 7.5 parts of polyethylene with a molecular weight of about 1,250,000 are heated together to above the fusion temperature of this polyethylene, a gel results which cannot be poured from the containing beaker. Extrusion, calendering or injection molding of this gel causes the polyethylene to undergo shear degradation and the resulting porous object loses much of its superior physical properties.
U.S. Pat. No. 3,607,793, dated Sept. 21, 1971, discloses the preparation of a porous sheet containing a particular polypropylene homopolymer or copolymer. The preparation involves the heating of a mixture of said polypropylene and a particular hydrocarbon to a temperature at which a homogeneous mixture is obtained. And then, the homogeneous mixture is formed into the desired structure, and cooled, and the hydrocarbon extracted.
U.S. Pat. No. 3,536,796, dated Oct. 27, 1970, discloses the preparation of a porous sheet containing an ethylene copolymer, a filler, a petroleum oil, stearic acid and an antioxidant. The preparation involves the extrusion of a melt mixture of the foregoing five components. As is known, extrusion is a shaping operation after the material is melted in the barrel of the extruder. Upon cooling of the shaped article the filler and/or oil is extracted.
U.S. Pat. No. 3,228,895, dated Jan. 11, 1966, discloses a method of making a microporous article from a mixture of a modified polyolefin and a plasticizer. The modified polyolefin is shown as having a measurable melt index whereas present invention is directed towards an ultra high molecular weight polyethylene having no measurable melt index.
U.S. Pat. No. 3,024,208, dated Mar. 6, 1962, discloses a process for the production of a porous polyethylene article. The polyethylene disclosed has a molecular weight ranging from 100,000 to 1,000,000. The process consists of sintering, that is, heating of particles under pressure without the actual melting of the particles.
U.S. Pat. No. 3,336,244, dated Aug. 15, 1967, discloses a porous product prepared by sintering a mixture of a polyolefin, a thermosetting resin and a different thermoplastic resin.